1. Field of the Invention
The present general inventive concept relates to a screen design method, and more particularly to a clustered dot-screen design method, a device to perform the clustered dot-screen design method based on human vision and printer model characteristics, and an image-forming apparatus to output binary images on a designed screen.
2. Description of the Related Art
In general, binary output devices, such as digital printers, photocopiers, binary output LCDs, and the like, express a diverse array of colors using two colors including white and black. For example, a black and white digital printer expresses black and white images displayed on a monitor using two values that respectively correspond to black and white. In order to output the black and white images having different brightness levels that are displayed on the monitor to the black and white digital printer, the black and white digital printer or a personal computer performs a series of operations to convert input images into binary images. That is, an operation for converting a color image into a gray-scaled image by converting respective pixel colors into gray colors having brightness of 0 to 255 is required. An operation for converting the gray-scaled image into a binary image is also required. Images having brightness values between 0 (black) and 255 (white) are referred to as gradation images, and the operation for converting the gradation images into the binary images is referred to as halftoning.
The halftoning technique includes error diffusion methods, which are mainly used in inkjet printers and multi-function printers, and screening methods, which are mainly used in laser printers.
The error diffusion methods effectively reproduce continuous gradation images while maintaining boundaries, since the error diffusion methods minimize an average error in the binary images by distributing errors, which occur during conversion of the continuous gradation images into the binary images, to neighboring pixels.
The screening methods convert the continuous gradation images into tiny dots in order to reproduce a shading or a gradation of the continuous gradation images.
The screening methods can be classified as a clustered-dot screening method for forming binary halftone dots as closely as possible or as a dispersed-dot screening method for forming the binary halftone dots as far away as possible. The clustered-dot screening method has a poor performance when displaying detailed image components as compared to the dispersed-dot screening method. However, the clustered-dot screening method advantageously has good tone reproduction and a high tolerance for drawbacks that typically occur in the laser printers. On the other hand, the dispersed-dot screening method is more suitable for display devices or the inkjet printers, since the dispersed-dot screening method has an advantage of being able to express the detailed image components effectively even though the dispersed-dot screening method is easily affected by the drawbacks that typically occur in the laser printers.
Other types of screening methods also exist. For example, an ordered-dot screening method regularly forms binary halftone dots depending on certain lines per inch (LPI) and angles, and a stochastic-dot screening method irregularly forms binary halftone dots. The stochastic-dot screening method has an advantage over the ordered-dot screening method with respect to image quality, since less visual patterns can be perceived when compared to the ordered-dot screening method. However, halftoned images produced by the ordered-dot screening method have a better compression rate.
Recently, interest in model-based halftoning has increased.
FIG. 1 is a flow chart illustrating a conventional halftoning method for designing a stochastic clustered-dot screen. The method of FIG. 1 is disclosed in U.S. Pat. No. 5,859,955.
The halftoning method illustrated in FIG. 1, which uses the stochastic-dot screening method and the clustered-dot screening method, designates a plurality of center dots without any specific directions or periodic patterns (operation S10). The halftoning method then determines a dot growth sequence about each of the plurality of center dots using a predetermined cost function (operation S20).
Another conventional method for designing a screen for outputting halftoned images is disclosed in U.S. Pat. No. 6,335,989. U.S. Pat. No. 6,335,989 discloses a halftoning printing method using donut filters, in which a screen is designed by using a cost function that is different from the cost function used in the above system (illustrated in FIG. 1). However, a considerable similarity exists between the above system (illustrated in FIG. 1) with respect to the dot growth sequence about the plurality of center dots without any specific direction and periodic patterns.
FIG. 2A is a view illustrating a halftoning result at 141 LPI (lines per inch) and a 45° screen angle designed by a conventional halftoning method, and FIG. 2B is a view illustrating a halftoning result at 212 LPI and a 45° screen angle designed by the conventional halftoning method.
Further, the conventional methods described above use the stochastic-dot screening method to reduce visual patterns in output halftoned images to obtain high-quality images. Further, the conventional methods described above use the clustered-dot screening method together to obtain stable outputs from imperfect output devices, such as the laser printers.
These conventional halftoning methods use momentum and distance penalty functions as cost functions to optimize dots about a center dot. However, the cost functions have a problem of insufficiency for obtaining optimal halftoned images, since the momentum and distance penalty functions do not take into account human vision characteristics and printer model characteristics.